This invention relates to rotatable assemblies for dynamoelectric machines, such as electronically commutated motors. More particularly, it relates to a rotatable assembly which uses a molded matrix between the shaft and the flux ring, and to a method of fabricating such assemblies.
In past designs of rotatable assemblies for dynamoelectric machines, such as, for example, electronically commutated motors, permanent magnets are typically adhesively bonded to the outer surface of a steel drum which is attached to a rotatable shaft. In some designs, the steel drum comprises a plurality of stacked steel laminations mounted on the outer surface of a steel shell. In many applications for electronically commutated motors, the rotors used therein employ the same designs as used for existing induction motor rotors and include aluminum induction motor bars. However, for electronically commutated motors and the like, neither the stacked steel laminations nor the aluminum bars serve any significantly beneficial electromagnetic function. For such applications, all that is needed in order to provide a medium of distribution for the magnetic flux generated by the magnets is a steel shell. Since eddy current losses are not a concern, the use of steel laminations is not necessary. Similarly, the aluminum bars are not necessary, and make the rotor more costly. Also, using a steel drum directly attached to the shaft is a relatively expensive use of materials for transmitting the electromagnetic torque generated by the magnets to the shaft, and further adds to the weight of the assembly. What is desired, then, is a light, inexpensive flux ring, a means for mechanically locking the permanent magnets to the ring, and a light, inexpensive means for transmitting torque from the flux ring to the rotor shaft.
Various schemes have been employed to retain magnet material elements, such as permanent magnets or the like, against displacement from their proper positions in a rotatable assembly for dynamoelectric machines. Once such scheme is described in U.S. Pat. No. 4,327,302, issued Apr. 27, 1982 to Doran D. Hershberger. In that patent, a plurality of magnet material elements in the form of bars are abutted between adjacent pole pieces of a rotatable assembly, with the magnet elements encapsulated in part by rotor bars cast between the adjacent pole pieces. The bars are integral with opposite end rings also cast on the rotatable assembly. Other patents, such as U.S. Pat. Nos. 4,242,610 and 3,531,670, describe heat shrinking a metal or metal alloy sleeve or band around the rotor core into displacement-preventing engagement with a set of magnet material elements, with the magnet elements arranged or otherwise seated in assembly positions about the circumference of the rotor core. While such an arrangement provides mechanical locking of the magnet elements to the rotor core, it requires undesirable heating during assembly, is also too bulky and cumbersome for some applications, and is relatively expensive. Also, for some dynamoelectric machines, the eddy current losses due to such metallic bands may significantly affect motor efficiency. Furthermore, if the retaining bands are formed from steel, they may create a short circuit of the magnetic flux, resulting in reduced torque output per unit of current input to the motor. U.S. Pat. No. 3,221,194 describes dipping a rotor core with permanent magnet material elements arranged thereon into a plastic bath. When cured, the plastic forms an encapsulating layer over both the rotor core and the magnet elements, securing them together. However, such a procedure also adds to the complexity of the assembly process.
Application Ser. No. 625,608, now U.S. Pat. No. 4,594,525, by the present inventor and assigned to the present assignee, discloses a rotatable assembly in which the magnet elements are mechanically locked to a generally cylindrically shaped flux ring. The flux ring with the magnet elements fastened thereto is attached to the rotor shaft by a plastic matrix molded between the shaft and the flux ring. The present invention provides a similar rotatable assembly, but employs a segmented flux ring in place of the continuous flux ring described in application Ser. No. 625,608, now U.S. Pat. No. 4,594,525. Employing a segmented flux ring provides the advantage that the rotatable assembly can be easily formed to provide the proper rotor-stator air gap, without requiring tight tolerances for the flux ring and magnet elements.
Accordingly, it is an object of the present invention to provide a method for making a rotor for a dynamoelectric machine which employs a light, inexpensive flux ring and a light, inexpensive means for transmitting torque from the flux ring to the rotor shaft.
It is another object of the present invention to provide a rotor in which the variations in radial magnet dimensions is compensated for to avoid rotor to stator air gap dissymmetries.